1. Field of the Invention
The present invention relates generally to the recovery of gold from refractory ores, and more particularly it relates to a novel treatment method which combines pressure oxidation of the ore, followed by cyanidation and carbon-in-pulp separation of the gold from the cyanidation pulp. The method is made practical by removal of heavy metals which were released during the pressure oxidation prior to cyanidation. Such heavy metals are cyanicides which can cause excessive cyanide consumption.
Conventionally, gold is recovered from gold bearing ores by cyanidation with a cyanide salt, typically sodium cyanide. Such treatment results in the formation of sodium aurocyanide, which is soluble and remains in solution until the gold is precipitated, typically with zinc. The precipitate is refined to yield the desired gold metal.
Unfortunately, many gold-bearing ores (referred to generally as "refractory ores") resist conventional cyanidation recovery, resulting in unacceptably high residual gold in the processed ore. A number of mechanisms may contribute to the refractory nature of a gold-bearing ore. For example, the gold may be occluded within various mineral formations, such as pyrites, arsenopyrites, sulfosalts, and the like. With such mineral locking, cyanide attack on the portion of the gold in the ore is prevented, and cyanidation is unable to solubilize that portion of the gold content. The gold ores including a significant amount of carbonaceous material are also known to be resistant to cyanidation. In some cases, the gold is chemically combined with the carbonaceous material in a manner which resists attack by the cyanide ion. Alternatively, the aurocyanide ion can be adsorbed onto the carbonaceous material during cyanidation, preventing separation from the ore. A third refractory mechanism occurs in clay ores, where the clay adsorbs the aurocyanide after it is released in the cyanidation step. Such clay adsorption inhibits ultimate separation of the aurocyanide ion from the treated ore slurry.
2. Description of the Prior Art
Various pretreatment processes have been proposed to release the gold from refractory ores prior to cyanidation. The use of chlorine or sodium hypochlorite generated in situ by electrolysis of brine has been taught to chemically oxidize carbonaceous ores. Scheiner et al., "Investigation of Oxidation Systems for Improving Gold Recovery from Carbonaceous Materials," Bureau of Mines Technical Progress Report (Heavy Metals Program), Department of Interior, July 1968. U.S. Pat. No. 2,777,764 to Hedley et al. discloses a pressure oxidation process for releasing gold which is occluded in sulfide minerals in the ore. The method consists of pretreating the ore in a non-alkaline oxidizing medium under pressure and at a high temperature. See also, U.S. Pat. No. 3,316,059 and 4,004,991 which concern the acid oxidation of zinc and iron ores. U.S. Pat. No. 4,289,532 describes an improved cyanidation gold recovery technique where the gold ore is oxidized and simultaneously subjected to cyanidation and counter current granular activated carbon adsorption in two or more stages.
Carbon-in-pulp processes for recovering gold values from very finely divided ore obtained from the grinding circuits have been used for some time. See, e.g., Hall (1974) World Min. 27:44-49, and Davidson et al. (1979) J. S. Afr. Inst. Min. Metall. 79:281-297.